Dual-threshold spread spectrum correlator

ABSTRACT

An apparatus for decoding a received spread spectrum signal having a data signal modulated with a pseudo-noise code and transmitted as an RF spread spectrum signal. A first reference register holds a first pseudo-noise signal, a second reference register holds a second pseudo-noise signal, and a receive-register holds a received spread spectrum signal. A first modulo adder adds each chip of the received spread spectrum signal with each respective chip of the first pseudo-noise signal, thereby generating a first plurality of chip-comparison signals. A first summer adds the first plurality of chip-comparison signals, generating a first correlation signal. The comparator compares the correlation signal to an upper threshold level and a lower-threshold level, and respectively generates a first data-symbol signal or a second data-symbol signal.

This application is a continuation of application Ser. No. 08/216,084filed Mar. 21, 1994, now U.S. Pat. No. 5,499,265 which is a continuationof application Ser. No. 07/949,331 filed Sep. 21, 1992, now abandoned,which is a continuation of application Ser. No. 07/698,458, filed May10, 1991, now abandoned, which is a continuation of application Ser. No.07/390,315, filed Aug. 7, 1989, now U.S. Pat. No. 5,022,047.

BACKGROUND OF THE INVENTION

This invention relates to spread spectrum communications, and moreparticularly to a non-code synchronous spread spectrum communicationssystem.

DESCRIPTION OF THE PRIOR ART

A spread spectrum system is one in which the signal is spread over aband much wider than the maximum bandwidth required to transmit theinformation being sent. Techniques for direct sequence spread spectrummodulation have been developed for several years to ensure securecommunications. Modulation is achieved by mixing the information to besent with a periodic pseudo-noise (PN) code. The result is a sin(X)/Xsignal with a very wide bandwidth, as compared to the information, andlow spectral density. This spectral density reduces the signal'ssensitivity to in-band interference and jamming, as well as reducinginterference with other radio sensitive equipment. Among the otheradvantages inherent to a spread spectrum system are selective addressingcapabilities, code division multiplexing for multiple access, and highlyaccurate ranging capabilities.

Due to the encoded nature of the signal, demodulation is a more involvedprocess than with traditional communications systems, and involves areference code, identical to that transmitted, synchronized to thereceived code. The difficulty with this process is that there is noindication of the degree of non-synchronization between received andreference codes until a very high degree of synchronization is achieved.Additionally, mismatches between transmit and receive oscillators usedto generate PN codes tend to cause drift in the synchronization betweentransmitter and receiver.

A prior art communications system using two pseudo-random waveforms andtwo correlators for designating a MARK or a SPACE, is disclosed in U.S.Pat. No. 4,247,942, to Hauer, issued Jan. 27, 1981, which isincorporated herein by reference. Hauer discloses in a communicationsystem, a first delay line having multiple spaced taps for supplyingsuccessive input pulses to the delay line. In response to each inputimpulse, variously delayed pulses appear at the taps of the delay line,which are used to generate pulses representing a MARK or a SPACE. Hisdisclosure includes synchronous detectors, and means for supplying thecarrier-transmitted pulses to the detectors.

None of the prior art teaches or suggests an apparatus for acquiring aspread spectrum signal using one correlator, nor an apparatus having anacquisition time of a spread spectrum signal equal to the time durationof one data bit on every data bit. Further, none of the prior artteaches dual threshold detection in a single correlator such that afirst data symbol and a second data symbol can be determined with equalaccuracy in one correlator, nor the use of a plurality of such dualthreshold detection correlators to achieve a doubling of the data ratefor each additional dual threshold detection correlator withoutincreasing the code rate or the signal acquisition time.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the invention is to provide an apparatus for acquiring aspread spectrum signal using one correlator which is direct,inexpensive, and simple to use.

Another object of the invention is to provide an apparatus for acquiringa spread spectrum signal that has a dual threshold detection capabilityin one correlator for determining the occurrence of a first data symbolor a second data symbol.

A still further object of the invention is to provide an apparatus fordetecting a spread spectrum signal without the use of a synchronousreference code.

An additional object of the invention is to provide an apparatus whichwill acquire a spread spectrum signal on each data bit received at therate the data is transmitted with no time loss due to codesynchronization and without use of any code synchronization preambles.

A further object of the invention is to provide the ability to use aplurality of dual threshold detection correlators (DTDCs).

Another object of the invention is to provide a plurality of DTDCs suchthat the transmitted and received data symbol rate can be increasedwithout increasing the transmitted and received code bit rate, norincreasing the bandwidth of the frequency spectrum which is utilized.

An additional object of the present invention is that for the pluralityof DTDCs, the transmitted and received data symbol rate can be increasedby a factor of two while the required number of correlators in thereceiver increases by an increment of one.

An additional object of the present invention is that for the pluralityof DTDCs, the time for acquiring the spread spectrum signal remainsconstant even as the transmitted and received data symbol rateincreases.

According to the present invention, as embodied and broadly describedherein, an apparatus for decoding a received spread spectrum signal,which includes a data signal modulated with a PN code, is provided,comprising threshold setting means, first reference-sequence-storagemeans, receive-sequence-storage means, first correlating means, andcomparison means. The first correlating means may include firstchip-comparison means and first summer means. The present invention mayadditionally include second reference-sequence-storage means, and secondcorrelating means. The second correlating means may include secondchip-comparison means and second summer means.

The threshold setting means may set the threshold for a match equal toor less than the total number of chips per code being acquired. Thethreshold setting means may analyze the pattern and application of thedata to be transmitted and/or received using the spread spectrum signal,the noise environment, and the amount of error correction used with thedata signal. In response to this analysis, the threshold setting meansgenerates a threshold level. The threshold setting means may be used topredetermine each threshold for a specific device , application, oroperating environment, or it may be coupled to the spread spectrumreceiver and user to set and adjust one or more threshold levels as theapplication or operating environment changes.

For the case of one correlator having a dual threshold capability, thefirst reference -sequence-storage means stores a first pseudo-noisesignal, and the receive-sequence-storage means stores the receivedspread spectrum signal. The first chip-comparison means, which iscoupled to the first reference-sequence-storage means and thereceived-sequence-storage means, compares each chip of the receivedspread spectrum signal with each respective chip of the firstpseudo-noise signal to generate a first plurality of chip-comparisonsignals. The first summer means, which is coupled to the firstchip-comparison means, adds the first plurality of chip-comparisonsignals and thereby generates a first correlation signal. For a digitalcorrelator implementation, in response to the first correlation signalbeing greater than the upper-threshold level, the comparison meansgenerates a first data-symbol signal. In response to the firstcorrelation signal being less than the lower-threshold level, thecomparison means generates a second data-symbol signal.

For an analog correlator implementation, in response to the firstcorrelation signal being greater than an analog-threshold level, thecomparison means generates a first data-symbol-correlation signal. Inresponse to the first inverse-correlation signal being greater than theanalog-threshold level, the comparison means generates a seconddata-symbol-correlation signal.

For the case of two correlators, each having a dual thresholdcapability, the first reference-sequence-storage means stores a firstpseudo-noise signal, the second reference-sequence-storage means storesa second pseudo-noise signal, and the receive-sequence-storage meansstores the received spread spectrum signal. The first chip-comparisonmeans is coupled to the first reference-sequence-storage means and thereceive register means. In response to the received spread spectrumsignal, the first chip-comparison means compares each chip of thereceived spread spectrum signal with each respective chip of the firstpseudo-noise signal and thereby generates a first plurality ofchip-comparison signals. The first summer means is coupled to the firstchip-comparison means. In response to the first plurality ofchip-comparison signals from the first chip-comparison means, the firstsummer means adds the first plurality of chip-comparison signals andthereby generates a first correlation signal.

The second chip-comparison means is coupled to the secondreference-sequence-storage means and the receive-sequence-storage means.In response to the received spread spectrum signal, the secondchip-comparison means compares each chip of the received spread spectrumsignal with each respective chip of the second pseudo-noise signal andthereby generates a second plurality of chip-comparison signals. Thesecond summer means is coupled to the second chip-comparison means. Inresponse to the second plurality of chip-comparison signals, the secondsummer means adds the second plurality of chip-comparison signals andthereby generates a second correlation signal.

The comparison means is coupled to the first summer means and the secondsummer means. The comparison means for the digital correlatorimplementation includes upper- and lower-threshold levels for eachreference-sequence-storage means. In response to the first correlationsignal being greater than the upper-threshold level, the comparisonmeans generates a first data-symbol signal. In response to the firstcorrelation signal being less than the lower-threshold level, thecomparison means generates a second data-symbol signal. In response tothe second correlation signal being less than the lower-threshold level,the comparison means generates a third data-symbol signal. In responseto the second correlation signal being greater than the upper-thresholdlevel, the comparison means generates a fourth data-symbol signal.

The comparison means for the analog correlator implementation includesfirst and second threshold levels, which may be equivalent for eachreference-sequence-storage means. In response to the first correlationsignal being greater than the fist threshold level, the comparison meansgenerates a first data-symbol signal. In response to the first invertedcorrelation signal being greater than the second threshold level, thecomparison means generates a second data-symbol signal. In response tothe second inverse correlation signal being greater than the secondthreshold level, the comparison means generates a third data-symbolsignal. In response to the second correlation signal being greater thanthe first threshold level, the comparison means generates a fourthdata-symbol signal.

Statistically, the accuracy of detection depends, in part, upon thethreshold level settings, which may be a function of several variables:The total number of matched chips to the total number of chips on a perdata symbol basis, the error rate and the degree of forward errorcorrection on the input signal, and whether the data stream to beprocessed is continuous, cyclically repetitive, patterned, episodic,pulsed or random.

The present invention may further include a plurality ofreference-sequence-storage means for storing a plurality of pseudo-noisesignals with means for correlating the received spread spectrum signalwith the plurality of pseudo-noise signals. The correlating means wouldthereby generate a plurality of correlation signals, depending on theparticular received spread spectrum signal. Likewise, the comparisonmeans would be responsive to the plurality of correlation signals forgenerating one of a plurality of data-symbol signals in response to thecorrelation signals crossing a particular threshold level in one of theDTDC reference-sequence-storage means.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate a preferred embodiment of theinvention, and together with the description serve to explain theprinciples of the invention.

FIG. 1 is a block diagram for a particular embodiment of a transmitteraccording to the present invention;

FIG. 2 is a timing diagram for a particular signal according to thepresent invention;

FIG. 3 illustrates one embodiment of a receiver according to the presentinvention;

FIG. 4 is a block diagram of a second embodiment of a transmitteraccording to the present invention;

FIG. 5 is a block diagram of a second embodiment of a receiver accordingto the present invention; and

FIG. 6 illustrates an analog embodiment of the present invention asexemplified by a SAW correlator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

The present invention includes an apparatus for transmitting anddecoding a received spread spectrum signal having a data signalmodulated with a PN code to produce a spread spectrum signal. Asillustratively shown in FIG. 1, a transmitter is shown including a codeclock generator 21, code selector 28, code generator 22, code clockdivider 23, pulse generator 24, data generator 25, modulo 2 adder 26,carrier 29, and phase shift keyed (PSK) modulator and RF transmitter 27.The code clock generator 21 generates a clock signal which is fed to thecode generator 22 and to the clock divider 23. Using clock divider 23,the code and the data of the transmitter 27 are synchronized, with thecode clock frequency being a multiple equal to the code length, L, ofthe data clock frequency thereby allowing one data bit per PN codesequence of length L. The data clock signal from code clock divider 23is fed to the pulse generator 24 and to the data generator 25. The datagenerator 25 is the source of the data signal to be transmitted over thecommunications system. The output signal of the pulse generator 24 isfed to the code generator 22 which thereby generates a PN code chosen bycode selector 28. The PN code, which is recursive, is then fed out ofthe code generator 22 and modulo 2 added with modulo 2 adder 26 to thedata fed from the data generator 25. The output of the modulo adder 26is the data signal modulated with the PN code, which is phase shiftkeyed modulated with a carrier 29 in PSK modulator and RF transmitter27.

An example of the timing of the code and clock signals of FIG. 1 isshown in FIG. 2. The timing diagram illustrates a code clock signal, adata clock signal which is the code clock signal divided by the lengthof code, L, pulse generator reset signal, and the code signal, the datasignal, and the encoded data signal. The code signal, when modulo 2added with the data signal produces the encoded data signal and whenmodulated by an RF carrier produces a spread spectrum signal. Thetransmitted spread spectrum signal may be received by the receiverillustrated in FIG. 3.

While FIG. 3 illustrates a particular embodiment of one dual thresholddetection correlator receiver of the present invention, the presentinvention in general includes an apparatus for decoding a receivedspread spectrum signal having a data signal modulated with a PN code.The apparatus comprises threshold means, firstreference-sequence-storage means, receive-sequence-storage means, firstcorrelating means, and comparison means. The first correlating means mayinclude first chip-comparison means and first summer means. Withreference to FIG. 3, the first reference-sequence-storage means may beembodied as reference registers 33, the receive-sequence-storage meansmay be embodied as receive registers 39, the first chip-comparison meansmay be embodied as first adders 40, the first summer means may beembodied as first summer 41, and the comparison means may be embodied asfirst and second symbol comparators 42, 43. The threshold setting meansmay be embodied as threshold setter device 45. The first adders 40 arecoupled to the first reference registers 33 and the receive registers39. First summer 41 is coupled to first adders 40.

In the exemplary arrangement shown in FIG. 3, a count control 30 may becoupled to a code clock 31 which is connected to a code generator 32 andreference registers 33. The code generator 32 is also connected to thereference registers 33. A code selection circuit 34 is coupled to codegenerator 32. The count control 30 controls the length of the particularpseudo-noise signal chosen by code selection circuit 34 to be detectedby the receiver, and outputs signals to the code clock generator 31which causes the code generator 32 to output a code of length L to firstreference registers 33. Count control 30 triggers code clock generator31 which thereby triggers code generator 32 and first referenceregisters 33. Code generator 32 outputs the particular pseudo-noisesignal to first reference registers 33 as determined by a code selectioncircuit 34. The code selection circuit 34 can provide signals to thecode generator 32 which enable it to scan through a plurality of pseudonoise codes. In operation, a single code can be loaded into the firstreference registers 33 or, in a scanning mode, the first referenceregisters 33 can be periodically loaded with constantly varying codesuntil a match to a received code occurs.

Also shown in FIG. 3 is RF and IF amplifiers 35, coupled to a productdetector 36 which is coupled to a local oscillator 37 and a low passfilter 38. The low pass filter 38 is coupled to receive registers 39 andclock recovery circuit 46.

For the case of one correlator having a dual threshold capability, thefirst reference registers 33 stores a first pseudo-noise signal, and thereceive registers 39 stores the received spread spectrum signal. Thefirst adders 40 compares each chip of the received spread spectrumsignal with each respective chip of the first pseudo-noise signal togenerate a first plurality of chip-comparison signals. The first summer41 adds the first plurality of chip-comparison signals and therebygenerates a first correlation signal. In response to the firstcorrelation signal being greater than the upper-threshold level, thecomparator 42 generates a first data-symbol signal. In response to thefirst correlation signal being less than the lower-threshold level, thecomparator 42 generates a second data-symbol signal.

In operation a received spread spectrum signal having a data signalmodulated with the PN code would be stored in receive registers 39 andthe entire length, L, of a first pseudo-noise signal would be stored infirst reference registers 33. Each chip of the received spread spectrumsignal is modulo 2 added by each respective chip of the first referencepseudo-noise signal by first adders 40. This modulo addition of the twosignals would thereby generate a first plurality of chip-comparisonsignals which would be transferred from first adders to first summer 41.The first summer 41 adds the first plurality of signals to generate afirst correlation signal.

The first symbol comparator 42 and second symbol comparator 43 arecoupled to the first summer 41. The comparators 42, 43 have anupper-threshold level and a lower-threshold level. In response to thefirst correlation signal being greater than the upper-threshold level,the fist symbol comparator 42 generates a first data-symbol-correlationsignal. In response to the first correlation signal being less than thelower-threshold level, the second symbol-comparator 43 generates asecond data-symbol-correlation signal. Data generator 47 therebygenerates first or second data symbols, per the first or seconddata-symbol-correlation signal, respectively. The first and seconddata-symbol signals may be, respectively, 1-bit and 0-bit data signals.

The present invention can further include two or more, up to N,different recursive sequences, with an example using two as illustratedin FIG. 4. In this particular case, a first code generator 52 and asecond code generator 53 each generate a difference recursive sequenceas chosen by code selector 62 for generating a spread spectrum codesignal. Code clock generator 51 feeds code clock to code generator A 52and code generator 53 as well as divider 54. Divide by L/2 feeds a clocksignal, which has a period equal to twice that of a code sequence ofLength L, to divide 2 56, two stage shift registers 58, and datagenerator 55. Thus, two data bits per code length L are generated bydata generator 55 and stored in parallel in two stage shift registers58. Divider 56 feeds a clock signal once per code segment of length L tosampler 57 and pulse generator 59 which resets code generator A 52 andcode generator 53. Sampler 57 outputs two signals to selector 60, whichdetermine which of four code segments (code A, inverse code A, code B,inverse code B) which it receives from code generator A 52 and codegenerator B 53. The modulated code is then transferred from selector 60to PSK modulator and RF transmitter 61 where it is PSK modulated with anRF carrier 63 in the transmitter. Transmitting a spread spectrum signalusing the circuitry of FIG. 4 has an advantage in that as shown in FIG.5, as a particular embodiment of the present invention, a receiver canbe implemented using only two correlators for detecting four possiblespread spectrum received codes. The particular codes are actually twospread spectrum PN codes, with 180° phase reversals.

Thus, the present invention can further include secondreference-sequence-storage means, second chip-comparison means, andsecond summer means. The second reference-sequence-storage means may beembodied as the second reference registers 73, the secondchip-comparison means may be embodied as second adders 80, and thesecond summer may be embodied as second summer 81. The secondreference-registers 73 stores a second pseudo-noise signal. The secondadders 80 are coupled to the second reference-registers 73 and thereceive registers 39.

For the case of two comparators as illustratively shown in FIG. 5, eachhaving a dual threshold capability, the first adders 40 compares eachchip of the received spread spectrum signal with each respective chip ofthe first pseudo-noise signal and thereby generates a first plurality ofchip-comparison signals. The first summer 41 is coupled to the firstadders 40. In response to the first plurality of chip-comparison signalsfrom the first adders 40, the first summer 41 adds the first pluralityof chip-comparison signals and thereby generates a first correlationsignal.

The second adders 80 are coupled to the second reference registers 73and the receive registers 39. In response to the received spreadspectrum signal, the second adders 80 compares each chip of the receivedspread spectrum signal with each respective chip of the secondpseudo-noise signal and thereby generates a second plurality ofchip-comparison signals. The second summer 81 is coupled to the secondadders 80. In response to the second plurality of chip comparisonsignals, the second summer 81 adds the second plurality ofchip-comparison signals and thereby generates a second correlationsignal.

In this particular embodiment, the comparison means is embodied ascomparison circuit 97. The comparison circuit 97 has upper- andlower-threshold levels. The comparison circuitry 97 is coupled to thefirst summer 41 and the second summer 81. In response to the firstcorrelation signal from the first summer 41 being greater than itsupper-threshold level the comparison circuitry 97 generates a firstdata-symbol-correlation signal. In response to the first correlationsignal from summer 41 being less than its lower-threshold level, thecomparison circuitry 97 generates a second data-symbol-correlationsignal. In response to the second correlation signal from second summer81 being greater than its upper-threshold level, comparison circuitry 97generates a third data-symbol-correlation signal. In response to thesecond correlation signal from second summer 81 being less than itslower-threshold level, the comparison circuitry 97 generates a fourthdata-symbol signal. Data generator 98 then generates the data symbolcorresponding to the received data-symbol-correlation signal. The first,second, third, and fourth data-symbol signals may represent, forexample, data bits 00, 01, 10, and 11.

While the present invention has disclosed using either a firstcorrelator as shown in FIG. 3 or a first and second correlator as shownin FIG. 5, the present invention may be extended to using a plurality ofcorrelators where the decoding decodes a plurality of data-symbolsignals.

By using a plurality of DTDCs in the receiver, a code of length L can beused to transmit and receive a number of data symbols equal to twice thenumber of DTDCs in the receiver. Another advantage of using a pluralityof DTDCs is that while the transmitted and received data symbol ratedoubles with each additional DTDC, the code length, L, the aggregatecode chip rate, and the bandwidth of the frequency spectrum used, allremain fixed. Moreover, the time to acquire the spread spectrum signalremains fixed, and the system remains inexpensive and simple. The codeselection circuit can provide signals to the code generators whichenable them to scan through a plurality of pseudo noise code. Inoperation, single code sets can be loaded into the reference registersor, in a scanning mode, the reference registers can be periodicallyloaded with constantly varying codes until a match to a received codeoccurs.

In the system provided by this invention, a pseudo-noise signal mayinclude a PN code segment having L bits produced with a clock rateequivalent to Lx(R_(d) /S_(d)), where R_(d) is the clock rate of thedata to be modulated, and S_(d) is the number of data bits per codesegment of length L. For example, if the data rate to be transmitted forone data bit is 100 kHz, and the code length, L, is 100, then the coderate, R_(c), is equivalent to Lx(R_(d) /S_(d))=(100)×(100 kHz/1)=10 MHz.For two data bits per code, the data rate rises to 200 kHz, but the coderate, R_(c) =(100)×(200 kHz/2)=10 MHz. Thus, the data rate can increasewithout a like increase in the code rate. The number of data symbolsproduced per code segment of length L is 2^(S) d where S_(d) =the numberof data bits per code segment.

In the transmitter, the beginning of a code segment of length L issynchronously aligned with each data set, defined by the number of databits per code segment, which may produce 2^(S) d data symbols. Forexample, if two data bits per code segment are to be transmitted, thenthe 2^(S) d=4 data symbols, which may be represented by 00, 01, 10, 11.If data symbol one is to be transmitted, then code one may be sent. Ifdata symbol two is to be transmitted then the inverse of code one may besent. The same process follows for data symbol three, which could usecode two's inverse, and data symbol four, which could use the normalcode two. In the receiver, reference code one and code two segmentsequivalent to those in the transmitter are loaded into the storageelements of the two correlators and held stationary. The received signalis then passed through the correlator, and when the correlator scorefrom correlator one exceeds its threshold value T, a firstdata-symbol-correlation signal is produced. If the correlation score isless than its L-T, then a second data-symbol-correlation signal isproduced. The same result follows for the third data symbol, when thesecond correlator score is less than its L-T, and the fourth data symbolwhen the second correlator score is greater than its T.

A second preferred embodiments of the present invention may use analogdevices such as surface-acoustic-wave (SAW) devices or charge-coupleddevices. The surface-acoustic-wave devices, as an example of analogdevices, include reference-sequence-storage devices andreceive-sequence-storage devices. The surface-acoustic-wave devicesadditionally may include the adders 40 and summers 41, and function as acomplete self-contained correlator unit. Additionally, a plurality ofsets of reference-sequence-storage devices may be constructed on oneparticular surface-acoustic-wave device along with thereceive-sequence-storage devices to form a very compact means fordecoding a plurality of pseudo-noise signals.

A delay line matched filter or SAW correlator is a passive devicedesigned to recognize a specific sequence of code chips, as does adigital correlator, but accomplishes this through a correlation of phaseshifts in an RF signal, rather than voltage levels at baseband, and cantherefore avoid many of the problems inherent in a digital correlator,such as a high noise or interference/jamming environment.

Each delay element within the correlator has a delay equal to the periodof the transmitted code clock such that each element corresponds to onlyone chip at any one time. As the received signal propagates down thedelay line, the phase structure of each element is added in or out ofphase with the propagated PN encoded wave, and the outputs of all theelements are then summed to arrive at a total correlation value. Whenall the phase shifts structures of the elements match the phase shiftsof the propagated value, then the maximum sum and correlation isachieved.

In order to achieve the desired correlation, the correct reference codemust be "loaded" onto the SAW device. The present discussion is for aBPSK device, however, the invention extends and includes any PSK such asMSK, QPSK, etc. Assuming a bi-phase shift keyed signal, phase reversalswould occur at each one/zero transition of the PN code. This is usuallyaccomplished in one of two ways. The first is through a programmablecorrelator which can output all phases in each element. Asillustratively shown in FIG. 6, for a bi-phase shift keyed device acount controller 101 controls a code clock generator 102 which sends Lclock signals to a code generator 104 and reference registers 105. Codegenerator 104 then produces a unique code as determined by code selector103 and loads it into reference registers 105. Once the code is storedin reference registers 105, the zero/one pattern is loaded into delayline correlator 106, with the contents of register A(2) connected toelement T(2), and so on to element A(L). The correlator is thenprogrammed so that all the outputs of the elements corresponding to afirst data symbol are connected to summing devices 108, 110 and alloutputs of the elements corresponding to a second data symbol areconnected to summing devices 109, 111. In this example, the first datasymbol is embodies as a first phase symbol, and the second data symbolis embodied as a second phase symbol.

In non-programmable devices, these phase shifts are programmed at thetime of construction through transducers placed in each element toproduce an elemental phase match and cannot be changed by the user, thusonly one code sequence can be correlated. Inverted and non-invertedphase elements are then summed together just as in the programmabledevice.

When a signal with a PN code, PSK modulation, and RF frequencyequivalent to that in the SAW correlator is received, then the receivedsignal is amplified (and maybe down-converted, although down conversionis to IF frequency is not preferred unless necessary) and fed to delayline correlator 106. As the wave propagates across the surface of thecorrelator, then energy in each delay element increases by a factordetermined by the phase of the reference elements versus the receivedsignal phase.

The output of the delay elements which are in phase with the first phaseare summed in summers 108, 110, while those elements which are 180° outof phase with the first phase are summed in summers 109, 111. When anon-inverted code segment, PSK modulated signal with the same PN codephase shifts as those referenced in the correlator propagates throughthe device and the first code chip reaches the end of the delay line allthe phase shifts of the received signal match those of the elementscomprising the correlator, and a first phase maximum energy is obtained.The output of inverted first phase summer 109 is inverted by phaseinverter 112 and summed in phase with the output of first phase summer108 in summer 114. If the output of summer 114 exceeds the threshold setin threshold detector 116 by threshold setter 118, a first data symbolcorrelation signal is generated by threshold detector 116 and fed todata generator 119, which produces a first data symbol signal.

When an inverted code segment, PSK modulated signal with the samenon-inverted PN code phase shifts as those referenced in the correlatorpropagates through the device and the first code chip reaches the end ofthe delay line, all the phase shifts of the received signal match thoseof the elements comprising the correlator, and an inverted first phasemaximum energy is obtained. The output of the first phase summer 110 isinverted by phase inverter 113 and summed in phase with the output ofinverted first phase summer 111 in summer 115. If the output of summer115 exceeds the threshold set in threshold detector 117 by thresholdsetter 118, a second data symbol correlation signal is generated bythreshold detector 117 and fed to data generator 119, which produces asecond data symbol signal.

A difference between the method and apparatus of this invention andthose used in the prior art is that the correlation pulse is used todirectly derive the data symbols, while other systems may use the pulsefor synchronizing a much longer reference code signal to the incomingreceived code signal.

A difference between SAW devices and digital correlators is in thefrequency bands in which they are used. The SAW devices are usuallyemployed at IF, but they can be used at RF. The digital correlators areusually used at baseband. Another difference is that SAW devices performphase shift comparisons while the digital correlators perform voltagelevel comparisons. Further, the SAW devices sum the outputs differentlyfrom that of digital correlators. Also, when the present invention isrealized with a SAW correlator, no receive code clock is required tocorrelate the PN code. The present invention, using a SAW correlator,may be realized using fewer components.

The present invention further includes methods using a correlator fordecoding a received PSK spread spectrum signal, which includes a datasignal modulated with a PN code, and modulated with an RF carrier toproduce a spread spectrum RF signal. The first method comprises ofsteps, using the digital correlator, of setting upper- andlower-threshold levels using a threshold setting means, storing apseudo-noise signal in a reference-sequence-storage means, storing thereceived spread spectrum signal in receive-sequence-storage means,correlating the received spread spectrum signal with the pseudo-noisesignal to generate a correlation signal, comparing the correlationsignal to an upper-threshold level and a lower-threshold level, andgenerating a first data symbol in response to the correlation signalbeing greater than the upper-threshold level, and generating a seconddata symbol in response to the correlation signal being less than thelower-threshold level.

The second method comprises, using an analog correlator as exemplifiedby a SAW correlator, of setting two threshold levels, which may beequivalent, using a threshold setting means, storing a pseudo-noisesignal in a reference-sequence-storage means, storing the receivedspread spectrum signal in receive-sequence-storage means, correlatingthe received spread spectrum signal with the pseudo-noise signal togenerate two correlation signals, comparing the first correlation signalto a first threshold level and comparing an inverse correlation signalto a second threshold level, and generating a first data symbol inresponse to the first correlation signal being greater than the firstthreshold level, and generating a second data symbol in response to theinverse correlation signal being greater than the second thresholdlevel.

It will be apparent to those skilled in the art that variousmodifications can be made to the apparatus for decoding a receivedspread spectrum signal, which includes a data signal modulated withspread spectrum, of the instant invention without departing from thescope or spirit of the invention, and it is intended that the presentinvention cover modifications and variations of the apparatus providedthey come within the scope of the appended claims and their equivalents.

We claim:
 1. A spread spectrum correlator, comprising:at least one receiving cache capable of buffering a received spread spectrum signal; at least one memory location capable of storing an entire code; at least one comparison circuit coupled with said at least one receiving cache and said at least one memory location, said at least one comparison circuit outputting a correlation signal; and a data generator that generates at least one data symbol in response to a comparison of the correlation signal with at least one predetermined threshold level.
 2. The spread spectrum correlator of claim 1, wherein said at least one comparison circuit comprises at least one adder and at least one summer, said at least one adder having an output connected to said at least one summer, said at least one summer outputting the correlation signal.
 3. The spread spectrum correlator of claim 1, wherein said data generator generates a first data symbol if the correlation signal exceeds a first predetermined threshold level, and said data generator generates a second data symbol if the correlation signal exceeds a second predetermined threshold level.
 4. The spread spectrum correlator of claim 1, wherein the code comprises a pseudo-noise code having a length of 100 chips.
 5. The spread spectrum correlator of claim 1, wherein said at least one memory location comprises first and second reference registers, said first reference register being capable of storing a first entire pseudo-noise code, said second reference register being capable of storing a second entire pseudo-noise code, and the second entire pseudo-noise code corresponding to a 180-degree phase reversal of the first entire pseudo-noise code. 